Basically a proportion is two equal fractions.  When you set them up with a word problem, you just need to be sure you are consistent.  For example:

The school cafeteria has found that they go through 8 sandwiches for every 5 tickets they sell.  If the have 325 tickets for sale, how many sandwiches do they need to make?

By consistency, I mean that you need to make sure the fractions on each side represent the same relationship.  I'm going to let mine be sandwiches over tickets, but it could just as easily be tickets over sandwiches...as long as you do both fractions or ratios the same way.

The first sentence says they sell 8 sandwiches for every 5 tickets, or 8 sandwiches per 5 tickets.  I can represent that ratio with the fraction

Now to be consistent I have to make my second fraction or ratio be sandwiches over tickets too. 

I know they sell 325 tickets so that will go on the bottom of the second fraction.

Since I don't know how many sandwiches go with the 325 tickets, I will pick a variable to represent that amount.  x is the most common choice.  That gives us

The procedure for solving a proportion is a technique called cross multiplying.  You multiply the top of one fraction by the bottom of the other and set those to products equal to each other.

To finish solving and find out what x is, divide both sides of the equation by 5.  That gives you

This reduces to x = 520 sandwiches.